The invention relates to a method for the highly accurate determination of the filling level of a product in a container, measuring signals being transmitted in the direction of the surface of the filled product and reflected at the surface as echo signals and the filling level in the container being determined by evaluation of the amplitude and phase values of the reflected echo signals by means of a pulse delay-time method. Both the pulse-radar method and the FMCW method, in which continuous waves are frequency-modulated in a periodically linear manner, for example with a sawtooth voltage, may be used. Furthermore, the invention relates to an apparatus for the highly accurate determination of the filling level of a product in a container.
Pulse delay-time methods use the physical law according to which the transit distance is equal to the product of the delay time and the propagation velocity. In the case of filling level measurement, the transit distance corresponds to twice the distance between the antenna and the surface of the filled product. The actual useful echo signal and its delay time are determined on the basis of the so-called echo function or the digital envelope curve, the envelope curve reproducing the amplitudes of the echo signals as a function of the xe2x80x98antenna-surface of the filled productxe2x80x99 distance. The filling level itself is then obtained from the difference between the known distance of the antenna from the bottom of the container and the distance of the surface of the filled product from the antenna, determined by the measurement.
DE 31 07 444 A1 provides a description of a high-resolution pulsed radar method. A generator generates first microwave pulses and transmits them via an antenna at a predetermined transmission repetition rate in the direction of the surface of the filled product. A further generator generates reference microwave pulses, which are identical to the first microwave pulses but differ slightly from them in the repetition rate. The echo signal and the reference signal are mixed. At the output of the mixer there is an intermediate-frequency signal. The intermediate-frequency signal has the same waveform as the echo signal, but is stretched in comparison with the latter by a time dilation factor which is equal to a quotient of the transmission repetition rate and the difference in frequency between the first microwave pulses and the reference microwave pulses. At a transmission repetition rate of several megahertz, a difference in frequency of a few hertz and a microwave frequency of several gigahertz, the frequency of the intermediate-frequency signal is below 100 kHz. The advantage of the transformation to the intermediate frequency is that relatively slow, and consequently low-cost, electronic components can be used for signal acquisition and/or signal evaluation.
The signal evaluation takes place by means of the so-called envelope-curve evaluation. The envelope curve itself is the result of a rectification, optionally a logarithmization and a digitization of the intermediate frequency. The distance is determined by determining the distance from a reference signal of the useful echo signal, which represents the filling level. The number of sampling points between the two maxima with a constant sampling time is directly proportional to the xe2x80x98antenna-surface of the filled productxe2x80x99 distance.
To increase the measuring accuracy, it has already become known to use for the evaluation not only the maxima (peaks), which supply amplitude information, but also their phase relationships. For this purpose, the amplitude-modulated intermediate frequency is demodulated and broken down into its complex elements. This is achieved, for example, by so-called quadrature demodulation, i.e. the intermediate frequency is multiplied once by a sine oscillation (Q) and once by a cosine oscillation (I), both oscillations having a frequency similar to the intermediate frequency. The high frequencies produced by the multiplication are filtered out with a low-pass filter. The envelope-curve signal HK is obtained from the root of the sum of the squares of I (in-phase component) and Q (quadrature component): HK=xc2x7{square root over (I2+L +Q2+L )}. Subsequently, the customary amplitude evaluation is carried out; at the found locations of the maxima, the respective phase relationship and the difference of the two phase relationships are additionally determined. The distance of the antenna from the surface of the filled product is made up of a component comprising integral wavelengths, resulting from the amplitude evaluation, and a phase remainder.
The known methods produce reliable results as long as it is ensured that the signals received by the antenna are reflected exclusively at the surface of the filled product (incidence and reflection take place in the direction of the normal). As soon as so-called multipath propagation occurs, the measuring accuracy deteriorates. Multipath propagation means that the echo signals contain not only the actual useful signal but also an interference signal component, which is attributable to retroreflections of the measuring signals at the container wall or at other internal elements located in the space inside the container.
Previous proposals for solving this problem are confined to excluding the occurrence of interference signals from the outset. According to a first configuration, undesired retroreflections can be prevented by the antenna being positioned so far away from the container wall (or some other interfering retroreflector) that the impingement of measuring signals on these xe2x80x98interference retroreflectorsxe2x80x99 can be reliably excluded.
The disadvantages of this possibility of avoidance are obvious; particularly with the considerable dimensions of the storage containers of the kind used in petrochemistry, for example, the retrofitting, repair or exchange of a filling-level measuring device in the central region of the container cover is much more difficult to accomplish than in the region of the edge. In addition to this there is a further source of error in the measurement of the filling level, the significance of which is all the greater the further the measuring device is away from the edge region of the container cover: the covers of large storage containers, such as storage tanks or silos, usually have an outwardly curved or tapered shape. If the side walls of the container bulge, which is always to be found in the region of the maximum filling level of the containers, the lowering of the cover, and consequently the changing of the xe2x80x98antenna-container bottomxe2x80x99 reference distance, has a greater effect in the central region than in the edge region.
Furthermore, it has previously been attempted to tackle the problem of multipath propagation by using an antenna with an optimized directional pattern. Such antennas, adjusted specifically for the individual case, are of course very expensive, which drives up the costs of the filling-level measuring device.
In certain applications it is necessary to replace the free-field measurement for determining the filling level by a measurement using a stilling well. Stilling wells are always used, for example, if the measurement results would be falsified by wave formation of the filled product within the container.
As already mentioned above, the filling level is determined by means of the delay time of the electromagnetic waves over twice the xe2x80x98antenna-surface of the filled productxe2x80x99 distance. In order that the measurement result is correct, the propagation velocity of the electromagnetic waves in the intermediate space between the antenna and the filled product must therefore be known exactly. In many applications, the propagation velocity is equated by approximation with the propagation velocity in air. However, this assumption is reliable only if the transverse dimensions of the space in which the electromagnetic waves propagate are large in comparison with the wavelength.
If the measurement of the filling level is performed by means of a stilling well, the assumption mentioned above is only approximately correct. A well or some other container of small diameter over which the waves propagate acts as a waveguide. Since the propagation of electromagnetic waves at the speed of light applies only in the ideal case where a wave packet can propagate undisturbed in free space, dispersion occurs in waveguidesxe2x80x94the propagation velocity is thus dependent on the wavelength. The group velocity in a waveguide, decisive for the propagation of a wave packet, is less than the speed of light.
If the wavelength is of the order of magnitude of the propagation space, a number of modes capable of propagation exist, having different propagation velocities on account of dispersion (xe2x86x92multimode propagation). The consequence of multimode propagation is the occurrence of constructive and destructive interferences between the measuring signals, which in turn leads to an oscillating measuring error being superposed on the actual filling-level measuring signal within a critical measuring range and falsifying the signal. Owing to the oscillation, the paradoxical case can occur that, in spite of an actual decrease in filling level for example, an increasing filling level value is indicated by the measuring apparatus. Such an obvious measuring error is of course completely unacceptable.
Incidentally, the critical measuring range lies between the transit distance of zero and a maximum transit distance, at which the measuring error disappears, since the wave packets have become so far apart that interferences no longer occur. This maximum transit distance is dependent on the nature of the waveguide and the frequency used for the electromagnetic waves.
This problem can again be alleviated by using waveguides of very high quality. In addition, the antenna must be adjusted optimally to the respective diameter of the waveguide. However, a low-cost solution is distinguished by the very fact that the wells do not have to be of a particularly high quality and that, furthermore, it is possible to use a low-cost antenna which is capable of covering any inside diameters of stilling wells acting as waveguides.
The invention is based on the object of proposing a method and an apparatus which permit highly accurate filling level measurements in containers-irrespective of the occurrence of multipath propagation or multimode propagation.
The object is achieved with respect to the method by compensation for errors which occur in the determination of the filling level and are caused by interference signals which interfere constructively or destructively with the echo signals reflected regularly at the surface of the filled product. Causes of errors which occur because of multipath propagation are constructive or destructive interferences between the actual useful echo signal that is reflected at the surface of the filled product and the component of the useful echo signal that is reflected by a retroreflector, for example the container wall. If xxe2x80x2 is the distance covered by the actual useful echo signal that has been reflected at the surface of the filled product, and xxe2x80x3 is the distance, longer by xcex94x, of the useful echo signal that has been reflected at some other retroreflector, in particular the container wall, interferences occur if the difference between the distances satisfies the condition xcex94x=nxc2x7xcex/2, n being any integral number. The maxima of the two signals are so close together that the resolution of the electronics is not adequate to separate the two peaks from one another. The method according to the invention now allows these errors to be reliably eliminated, which expressed in figures means that the error tolerances in the range of the filling level measurement are reduced by an order of magnitude in comparison with the previously known methods. The same problem also occurs, as described above, as a consequence of the multimode propagation of wave packets in stilling wells or other containers carrying the wave packets. The method according to the invention is therefore also ideal for eliminating measuring errors caused by multimode propagation.
The advantages of the solution according to the invention can be seen on the one hand in that the antenna can also be positioned in the edge region of the container or in the region of some other retroreflector located in the space inside the container without interference signals falsifying the measurement results. It is consequently now possible without any problem to go below the previously required minimum distance from an interfering reflector. Since it is possible for the component comprising the interference signals to be specifically eliminated from the useful echo signal, it is additionally possible to use an antenna which, although capable of being adapted optimally to the reflection properties of the respective filled product, does not need to have an optimized directional pattern dependent on the respectively desired installation position.
In the case of multimode propagation, the advantage likewise lies in the considerable cost savings, since on the one hand high-quality waveguides do not have to be used; on the other hand, one antenna can be used for a large number of waveguides or stilling wells with different dimensions.
According to an advantageous development of the method according to the invention, it is provided that a correction value is determined and stored for selected filling level values, at least within a critical filling-level measuring range in which the interference signals are produced.
A preferred embodiment of the method provides in particular that the starting filling level is measured; the correction value is subsequently initialized at zero; starting from this starting filling level, at least the critical filling-level measuring range is run through, for example successively and step-by-step; then a correction value is calculated and stored for every selected filling level value within the filling-level measuring range.
To allow the number of values actually measured within the critical filling-level measuring range to be reduced to a minimum amount, an advantageous embodiment of the method according to the invention proposes that, for providing a continuous set of correction values, the correction values between two, for example successive, filling level values are respectively interpolated.
A particularly advantageous embodiment of the method according to the invention provides that, for successive filling level values qK, qK+1, the correction values are determined as follows: let qK=q0 be the starting filling level. For the starting filling level, the correction value is initialized at a predetermined value, for example zero; subsequently, for the changed filling level qK+1, the difference of the corrected amplitude distance values Axe2x80x2 (qk) and the phase distance value is calculated. Consequently, a new correction value is determined and stored for each filling level successively addressed. The calculation of the new correction value is preferably performed in accordance with the following formula:
K(qK+1)="psgr"(qK+1)xe2x88x92A(qK+1)="psgr"(qK+1)xe2x88x92A(qK+1)xe2x88x92K(qK+1)
The embodiment according to the invention uses the fact that phase evaluation is much more sensitive in broad ranges, and consequently produces more accurate measurement results, than amplitude evaluation. If relatively great measuring errors occur in amplitude evaluation just because of multipath propagation or multimode propagation, the measuring errors which phase evaluation produces are still relatively small. Only if the measuring error in amplitude evaluation is greater than xcex/4 (which, for example at a frequency of 6.3 GHz, corresponds to a measuring error of 12 mm), does a phase shift occur in the phase evaluation, causing a measuring error of 24 mm. The error from the phase evaluation in the range of a phase shift is consequently twice the magnitude of the error from the amplitude evaluation. Since the method according to the invention avoids such phase shifts, the high measuring accuracy aimed for, of about 1 mm, in the determination of the filling level is achieved.
If a correction value is assigned to every filling level within the measuring range in which multipath propagation or multimode propagation is to be feared, it is subsequently possible to correct measured filling level values correspondingly. The previously critical case where the filling-level measuring device is switched off for some reason then does not present any problems. If all the correction values in the critical filling-level measuring range are known, that correction value which has previously been determined and stored for the corresponding filling level is used after switching on the measuring device for correcting the currently measured filling level.
Nevertheless, it may happen that the measuring device is switched off at a point in time at which the correction values are not yet available over the entire critical filling-level measuring range. Here, a distinction must be drawn between two cases: if it is ensured that the filling level has not changed during the switched-off period, an advantageous embodiment of the method according to the invention provides that the correction value last stored before switching off is used for initialization.
In the case where not all the correction values in the critical filling-level measuring range are known and, in addition, it is not possible to rule out the possibility that the filling level has changed during the switched-off period, the correction value is initialized at zero after the filling-level measuring device is switched off; for each correction value there is additionally stored a status value, which provides information on the reliability of the respective correction value. These status values may, for example, say: xe2x80x98certainxe2x80x99, xe2x80x98uncertainxe2x80x99, xe2x80x98undeterminedxe2x80x99. In the course of further measurements, the correction values determined are adapted to the correction values already stored, the newly determined correction values being corrected if need be.
One variant of the method according to the invention provides that a unique phase correction value (P(q)) is determined on the basis of the determined correction value, where P=nxc2x7xcex/2 with nxcex5 Z, and the phase distance value is corrected in a way corresponding to the unique correction value. It is further proposed that gray zones are defined, at which no unique phase correction value is defined after switching off and the correction value (K(q)) is small.